A new 16 seer Goodman heat pump was installed with the recommended indoor coil unit. The indoor unit was installed in an existing auxillary furnace. The unit works well but has begun to vibrate and hum when the outdoor compressor unit is running. What is causing it to vibrate and hum?????

Is the vibration coming from the furnace coil mounting? Could be a loose screw. Is is from the furnace motor? Could be a bearing going bad. Are the pipes insulated and hung tight? Might have a vibration from the coolant flow.